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I am not a regrigeration technician, but I have gone through this situation, and I offer my opinion. It is worth at least the price I ask for it, so use your judgment.
I will assume that the "Kit" you buy has a hose with Quick
Disconnect at one end and a threaded fitting on the other end that fits
either the Gauge OR the Valve, but NOT both at the same time (Professional gauge sets have TWO gauges and can have the gauge AND the Valve/Refrigerant connected at the same time).
1) If your system has completely lost its charge, and air has entered the system, then DO let a shop do the job if you've never done it and/or don't have the right equipment.
2) If your system is just down a bit on refrigerant (You say you already have an R-134 system), then air has NOT entered your system (since internal pressure is still WAY above atmospheric pressure), so there is no compelling reason to evacuate your system IMHO (unless you have a significant leak you wish to troubleshoot that will require "breaking" the system open).
A) Just buy the kit (get the one with the gauge) and hook it up as directed on the low pressure side (the one on the suction side of the compressor pump). It will read about 100-125 psi with a partly filled system with the engine not
running.
B) Start the car and turn the AC on max cool and read the gauge. If it's over 50psi, then you have a full charge and your problem is elsewhere (bad expansion valve, bad low pressure switch, bad clutch, bad compressor).
If your pressure reads under 40psi, then you can add some R-134.
Do this by:
1) Disconnecting the hose from the low pressure fitting.
2) Unscrew the gauge from the hose.
3) Connect the valve securely to the can of R-134 (MAKE SURE THE VALVE
IS SCREWED FULLY COUNTER CLOCKWISE-Unscrewed)
4) Connect the valve and can to the hose.
5) Connect the hose to the low pressure fitting again.
6) Start engine w/ AC on high.
7) Screw valve completely clockwise (to puncture can seal), then
counter clockwise to release R-134 from can. The can should be
held upright if only regrigerant is used, or inverted if you
are using R-134 with oil/leak check/O-Ring conditioner added.
The can should get quite cold as its internal pressure drops
(This is why the AC system works, by the way).
8) If you read approx. 25psi initially, add a full can (12-15 oz)
then recheck pressure as above. Be sure to disconnect the hose
from the fitting before unscrewing the valve and replacing it
with the gauge.
If you read close to 40psi initially, add approx
1/2 can of R-134 (close the valve fully when the can "feels"
about half empty and recheck the pressure. When the pressure
reads about 45psi, add no more R-134. You can save the remainder
of the can for later use as long as the valve is closed tightly.
I hope that helps some. If your system loses efficiency over a winter on non-use, or over a few months of summer use, you probably have a very small leak. You would have to spend about $75 at most shops just to have gas added (as above), or considerably more ($75-500+) to have the small leak diagnosed and repaired.
I see adding a can of refrigerant occasionally (at about $5 for 12oz) yourself as a cost effective way of maintaining a useable AC system. R-134 is supposedly safe for the environment, so losing a few oz. per month is not a disaster IMHO.
Good luck and let us know what you end up doing.
Bob
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